1. Field
The present invention relates to solid oxide electrochemical cells such as, e.g., a solid oxide fuel cell (SOFC) and a solid electrolyte high-temperature steam electrolysis cell (SOEC), fuel electrodes for use in the cells, and processes for producing the electrodes.
2. Description of the Related Art
A solid oxide fuel cell (SOFC) has an operating temperature as high as about 700° C.-1,000° C. and hence a high power generation efficiency, and is reduced in CO2 generation. This fuel cell is hopeful as a next-generation clean power generation system.
With respect to fuel electrode materials for solid electrolyte fuel cells, a technique has been disclosed in which SDC (CeO2 doped with Sm2O3) particles having electron/oxygen ion mixed conductivity are used to highly dispersedly deposit fine particles of nickel on the surface of the SDC particles (see J. Electrochem. Soc., 141, [2], 342-346, 1994).
In this technique, nickel particles are formed in a porous material constituted of an SDC network by the impregnation method using, e.g., an aqueous metal salt solution. This technique has succeeded in reducing the size of nickel particles by at least one order of magnitude and in obtaining high catalytic activity with a smaller nickel addition amount. In addition, since SDC further has electronic conductivity, the boundary between each of all the fine nickel particles and the SDC theoretically functions as a three-phase interface. There is a description in that document to the effect that bonding between the nickel and the SDC is relatively satisfactory. However, because nickel particles are formed by impregnation with the solution, burning, and reduction, the particles change in size with time or the particles unite/aggregate with one another during the burning step, resulting in an uneven structure.
Furthermore, techniques concerning the use of NiAl2O4 in a fuel electrode have been disclosed, which include the use of a system including NiAl2O4 and NiO (see Japanese Patent No. 3327789) and the use of a system including nickel as a main component and further including NiAl2O4 (see JP-A 2003-242985). Moreover, a technique in which a solid solution between NiO and MgAl2O4 is mixed with YSZ to form an electrode and this electrode is reduced to precipitate nickel has been disclosed (see JP-A 7-105956). There also is a technique in which an NiO—MgO solid solution is used in a fuel electrode (see JP-A 6-111829).
For reducing overvoltage and increasing catalytic activity in a fuel electrode, it is necessary to use finer metal particles as a catalyst and thereby increase the number of active sites. However, in a high-temperature reducing atmosphere, the metal particles are apt to readily move, grow, and aggregate. Furthermore, it is difficult to incorporate nickel particles in an unnecessarily large amount partly because of a difference in the coefficient of thermal expansion. In addition, in case where abrupt oxidation has occurred, the formation of an oxide results in volume expansion and leads to the fear of causing cell breakage.